In a fire, the glass sheet or pane of a conventional window unit is subjected to uneven heating because its edge is enclosed in the window frame and therefore not heated at the same rate as the center region of the sheet. As a result, stresses resulting from expansion occur in the edge region of the sheet and lead to rupture of the window.
Windows constructed to withstand the effects of fire without premature breakage are generally known. Examples of such windows are described in German Auslegungsschrifts Nos. 2,328,737 and 2,344,459 and German application No. 2,527,134. In these prior art constructions, provisions have been made for decreasing the temperature gradient between the center of the glass sheet and its edge region by leaving the edge of the glass sheet exposed, or at least partially exposed, on the side of the window unit which may be subjected to the effects of fire. Other constructions involve the use of protective glass placed on the fireside of the silicate glass or the use of glass which itself is specially constructed to resist the effects of fire.
In copending application "Fire-Proof Window", filed Dec. 5, 1977, Ser. No. 857,417, and assigned to the assignee of the present application, still another fire-proof window is disclosed. There, the edges of the glass sheet are covered with a decorative listel. The listel is adhesively secured, either directly or indirectly to the glass sheet by an adhesive or solder that loses its effectiveness upon being subjected to the heat of a fire. Thus, in a fire the listel falls away to expose the edges of the glass sheet to heating at substantially the same rate as the center of the sheet to prevent the creation of thermal stresses.